Single wafer plasma etch reactor

ABSTRACT

A high pressure, high etch rate single wafer plasma reactor having a fluid cooled upper electrode including a plurality of small diameter holes or passages therethrough to provide uniform reactive gas distribution over the surface of a wafer to be etched. A fluid cooled lower electrode is spaced from the upper electrode to provide an aspect ratio (wafer diameter: spacing) greater than about 25, and includes an insulating ring at its upper surface. The insulating ring protrudes above the exposed surface of the lower electrode to control the electrode spacing and to provide a plasma confinement region whereby substantially all of the RF power is dissipated by the wafer. A plurality of spaced apart, radially extending passages through the insulating ring provide a means of uniformly exhausting the reactive gas from the plasma confinement region. Affixed to the upper electrode is a first housing which supplies reactive gas and cooling fluid, and a baffle affixed to the first housing intermediate the upper electrode and a gas inlet forms a plenum above the upper electrode and ensures uniform reactive gas distribution thereover. The first housing and upper electrode are contained within a second housing with an insulating housing therebetween. The upper and lower electrodes are electrically isolated from each other and from ground, so that either or both electrodes may be powered.

BACKGROUND OF THE INVENTION

The present invention relates generally to apparatus for plasma etchingsemiconductor wafers and more particularly to a high pressure, high etchrate reactor for etching a single semiconductor wafer.

In the fabrication of semiconductor devices, the use of plasma etchinghas several advantages over conventional wet etching. Among these areanisotropy, better resolution, and the elimination of problems inherentin the use of wet etch chemicals. Further, the use of single wafer,plasma etching systems offers the advantages of higher etch rates andimproved etch uniformity as compared to multiple wafer or batch-typereactors. As linewidths decrease, for example, in very large scaleintegrated circuit devices, etch uniformity, or achieving a uniform etchrate across the surface of the wafer, and wafer-to-wafer becomes evenmore critical because of the increased number of devices on each wafer.

Single wafer, parallel plate plasma reactors are disclosed in U.S. Pat.No. 4,209,357 to Gorin et al., entitled "Plasma Reactor Apparatus", andU.S. Pat. No. 4,324,611 to Vogel et al., entitled "Process and GasMixture for Etching Silicon Dioxide and Silicon Nitride". The latterpatent describes the use of secondary and tertiary reactive gases in aconventional reactor, and the former patent describes a reactor whereinthe upper electrode includes both reactive gas supply means and exhaustmeans. In U.S. Pat. No. 4,297,162 to Mundt et al., entitled "PlasmaEtching Using Improved Electrode", the reactor includes a curved upperelectrode which is said to improve etch uniformity across the surface ofthe wafer. In some of these systems a wafer is fully exposed to theupper electrode, in other systems, such as the one disclosed in U.S.Pat. No. 4,367,114, an insulating ring is used as a partial confiningwall and clamps the wafer to the lower electrode to ensure electricalcontact. In such systems, the edge portion of the wafer that is clampedcannot be used for the fabrication of integrated circuit devices.

Current low (<100 microns) and medium pressure (<500 microns) plasmasystems have exhibited relatively low etch rates and correspondinglylong etch times, particularly when etching films greater than about onemicron in thickness. In addition, when reactive gas pressure and RFpower are increased in an attemmpt to increase the etch rate, the etchuniformity is degraded with an accompanying decrease in yield.

SUMMARY OF THE INVENTION

To achieve high etch rates with good uniformity it is necessary togenerate a uniform high density plasma over the wafer. One method is touse a very small reactor volume, e.g., by employing a smallinter-electrode gap for a given wafer size, so that for an input ofreasonably low area power density, high volume power densities can beobtained. One result of low inter-electrode spacing or high aspect ratio(wafer diameter:spacing) is the efficient use of secondary electronswhich cause many more surface and gas collisions, before they are lostto the walls, as compared to when the aspect ratio is small. In such acase, the desirable condition, where most of the input power is used inproducing plasma of low impedance rather than one with high sheathvoltage, is obtained. The critical factors in the fabrication of such areactor include electrode parallelism at low inter-electrode spacings,efficient wafer cooling, confinement of plasma over the wafer tominimize RF and plasma leaks away from the reaction zone, uniform gasdistribution and pump-out, and minimized RF and gas flow disturbancesaround the edge of the wafer.

Accordingly, the present invention overcomes many of the disadvantagesof prior systems by incorporating the above features to provide a singlewafer, high pressure, high plasma density, high etch rate parallel platereactive plasma etching system with improved etch uniformity and withoutresist degradation at high volume power densities.

In one embodiment of the present reactor, a fluid cooled upper electrodeincludes a plurality of small diameter holes or passages therethrough toprovide uniform reactive gas distribution over the surface of a wafer tobe etched. A fluid cooled lower electrode is spaced less than about 4mm. from the upper electrode and includes an insulating ring at itsupper surface. For a 125 mm. diameter wafer, a 4 mm. spacing results inan aspect ratio of about 31. The insulating ring protrudes above theexposed surface of the lower electrode to control the electrode spacingand to provide a plasma confinement region whereby substantially all ofthe RF power is dissipated by the wafer. A plurality of spaced apart,radially extending passages through the insulating ring provide a meansof uniformly exhausting the reactive gas from the plasma confinementregion. Affixed to the upper electrode is a first housing which suppliesreactive gas and cooling fluid, and a baffle affixed to the firsthousing intermediate the upper electrode and a gas inlet forms a plenumabove the upper electrode and ensures uniform reactive gas distributionthereover. The first housing and upper electrode are contained within asecond housing with an insulating housing therebetween. The upper andlower electrodes are electrically isolated from each other and fromground, so that either or both electrodes may be powered.

It is therefore an object of the present invention to provide a highpressure, high etch rate plasma reactor with improved etch uniformity.

Another object of this invention is to provide an improved highpressure, single wafer plasma reactor having fluid cooled upper andlower electrodes in a parallel plate configuration to facilitate coolingof the wafer from both sides.

Yet another object of the present invention is to provide a single waferplasma reactor wherein the electrode spacing is adjustable such that anaspect ratio of 25 can be maintained, and wherein high reactive gaspressure and high volume density RF power are combined to achieve highplasma density etch rate without sacrificing etch uniformity, andwithout causing resist degradation.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and many of the attendant advantages of the presentinvention will be better understood by reference to the followingdetailed description in conjunction with the accompanying drawings, inwhich like reference numerals designate the same or similar partsthroughout the several views, and wherein:

FIG. 1 is a cross-sectional view in elevation of a plasma reactoraccording to one embodiment of the present invention;

FIG. 2 is a top plan view of the plasma reactor upper electrode;

FIG. 3 is a cross-sectional view in elevation of a portion of the upperelectrode taken along line 3--3 in FIG. 2;

FIG. 4 is a cross-sectional view in elevation of another portion of theupper electrode taken along line 4--4 in FIG. 2;

FIG. 5 is a top cross-sectional view of the upper electrode taken alongline 5--5 in FIG. 6, showing the details of the internal coolingpassage;

FIG. 6 is a side view of the upper electrode of FIG. 2;

FIG. 7 is a cross-sectional side view of the plasma reactor innerhousing of FIG. 1, taken along line 7--7 in FIG. 8; and

FIG. 8 is a bottom plan view of the inner housing as seen from theperspective of line 8--8 in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, there is shown in FIG. 1 a single waferplasma reactor 10, according to one embodiment of the present invention,wherein a circular, electrically conductive upper electrode 12 isaffixed to a cylindrical inner housing 14. Housing 14 is conductive andincludes a perforated baffle 16 affixed thereto which forms a plenum tofacilitate the uniform distribution of a reactive gas over electrode 12.Housing 14 also includes a reactive gas inlet passageway 18, and coolingfluid inlet 20 and outlet 22 passageways (only outlet passageway 22 isvisible in FIG. 1) for supplying cooling fluid to and returning thefluid from upper electrode 12, as will be described in more detailbelow. The electrode 12 and inner housing 14 assembly is containedwithin an insulating housing formed of insulator 24, bushing 26, andexhaust and spacing ring 28 to provide electrical isolation of the upperelectrode assembly and set the inter-electrode gap. Surroundinginsulator 24 is a cylindrical outer housing 30 which contacts bushing 26and is affixed to exhaust ring 28, but is spaced apart from insulator 24to provide a gap 32 therebetween to transfer reactive gas to an exhaustassembly 34 at the upper portion of outer housing 30. Housing 30 may beeither conductive or non-conductive and is affixed to exhaust andspacing ring 28 by means of a clamp ring 36. The entire reactor assemblyis held and positioned with respect to a reference surface by a ringclamp 11.

Reactor 10 includes a lower electrode assembly 38 which is formed in oneembodiment of a conductive upper section 40 and an insulating lowersection 42 to provide electrical isolation. Upper section 40 has acentral raised portion which is surrounded by an insulating ring 44having a flange or channel formed into its uppermost inner edge. In thismanner the vertical inner surface 43 of ring 44 is spaced apart from theexposed surface 41 of electrode 40, while the horizontal surface 45 ofthe inner channel of ring 44 is displaced below the upper surface ofelectrode 40. An optional peripheral groove 39 may be formed ininsulating ring 44 to provide a means for auto-positioning a wafer onlower electrode 40. During transfer, when auto-transport is used, thewafer is supported on surface 45 of ring 44 while the ring is separatedfrom upper section 40 of electrode 38 by wafer handling apparatus (notshown). In the case where auto loading and unloading of a wafer is notused, the ring 44, without the groove 39, remains fixed to theconductive portion 40 of the lower electrode 38. When reactor 10 is inthe closed position as shown in FIG. 1, the depressed surface of theconductive section 40 of lower electrode 38 is pressed against theexhaust and spacing ring 28, and insulating ring 44 is in contact withupper electrode 12. The protrusion of the upper surface of ring 44 abovethe exposed surface 41 of lower electrode section 40 forms an enclosedplasma confinement region 46 between upper electrode 12 and surface 41.The spacing and parallelism between upper electrode 12 and lowerelectrode 38 which is set by the thickness of ring 28 is preferably lessthan about 4 mm to increase the volume power density in region 46.Alternatively, a 10-15 mil gap above ring 44, instead of passageway 48will also confine the plasma, though not very efficiently. Insulatingring 44 and exhaust ring 28 include radially extending passageways 48and 50, respectively, therethrough to provide means for exhaustingreactive gas from plasma region 46. Ring 28, in addition to passageways50, may have an exhaust manifold or groove (not shown) formed into itsinner surface so that the passageways 48 and 50 need not be aligned toeffectively exhaust the gases. The passageways 50 in exhaust ring 28open into the gap 32 between insulator 24 and outer housing 30. Thereactive gas thus enters through inlet 18, then passes through baffle 16and upper electrode 12 into the plasma region 46, through passageways 48and 50, through gap 32 and out exhaust assembly 34 in outer housing 30.

The lower surface of the upper section 40 of lower electrode 38 has achannel 52 formed therein which, when sections 40 and 42 are assembled,forms an enclosed passageway for directing cooling fluid therethrough.Cooling channel 52 has, in one embodiment, a serpentine configurationwhich forms a single continuous passageway. The cooling fluid may besupplied by the same source that supplies upper electrode 12, orseparate cooling fluid sources may be employed. In this manner, theoperating temperatures of electrodes 12 and 38 may be precisely andindividually controlled.

Electrodes 12 and 38 are electrically isolated from each other and areungrounded, therefore, either or both electrodes may be coupled to asource of RF power (not shown), with the other electrode being groundedduring operation. When powering the upper electrode, to avoid plasmabeing sustained in the annular space 32, a ground shield may cover theentire outer surface of the insulator housing 24. In this arrangement,the reactor can also conveniently be operated in triode, frequencymixing, and substrate tuning modes wherein both electrodes are powered.A semiconductor wafer to be etched is placed onto lower electrode 38 byeither raising the upper electrode assembly which includes electrode 12and housings 14, 24 and 30, or by lowering lower electrode 38.Conventional wafer handling apparatus may be employed in eitherinstance.

FIGS. 2-6 show upper electrode 12 in greater detail. Referring to FIG.2, electrode 12 includes a plurality of spaced apart openings orpassageways 54 extending vertically therethrough. In the presentembodiment, as shown in FIGS. 3 and 4, the upper portion of eachpassageway 54 is of one diameter while the lower portion is of a smallerdiameter. This facilitates fabrication of electrode 12 because it isdifficult to form a small diameter hole, for example, on the order ofabout 0.1-0.4 mm., entirely through the electrode, which may have athickness of about 25 mm. The small openings of passageways 54 at thelower surface of electrode 12 are necessary in order to uniformlydistribute a reactive gas over the surface of a wafer therebeneath, andto prevent the plasma from forming inside the passageways.

FIG. 5 is a top cross-sectional view of electrode 12 showing thestructure of a cooling passageway 56 formed therein. Referring also toFIG. 6, passageway 56 may conveniently be fabricated by forming aplurality of parallel, spaced apart holes 58 extending horizontallythrough electrode 12. A peripheral groove 60 is formed into electrode 12coinciding with the holes 58, and a portion of the region betweensuccessive holes 58 is removed at the innermost surface of groove 60.Alternate open regions 62 are thus formed at the surface of groove 60 asshown in FIG. 5. In the final assembly of electrode 12, a ring 64 (shownin FIG. 1) is fitted into groove 60 and welded to close the ends ofalternate holes 58. Alternatively, instead of the cross-section of ring64 shown in FIG. 1, a ring may be provided whose outer surface is flushwith the periphery of electrode 12 so that the outer surface ofelectrode 12 is smooth. With ring 64 inserted in groove 60, a single,continuous serpentine passageway 56 is formed in electrode 12 to providefor the flow of cooling fluid therethrough. Referring to FIG. 4, thecooling fluid is transferred to and from passageway 56 by a verticalinlet hole 66 and a similar outlet hole 68 which intersect the extremeends of passageway 56 as shown in FIG. 2, and indicated by dashed linesin FIG. 5. Uniform distribution of the cooling fluid through electrode12 is thereby provided.

Referring now to FIG. 7, there is shown a crosssectional side view ofthe inner housing 14 of FIG. 1. FIG. 8 is a bottom plan view as seenfrom the perspective of lines 8--8 in FIG. 7. Bell-shaped housing 14 hasan extended upper portion 70 and a wider lower portion 72. Upper portion70 has the vertical reactive gas passage 18 and cooling fluid passages20, 22 formed therein. Gas inlet passage 18 extends to the lower surfaceof housing 14, while cooling fluid inlet and outlet passages 20 and 22intersect horizontal passages 74 and 76, respectively, formed into lowerportion 72. Passages 74 and 76 then intersect vertical passages 78 and80, respectively, which extend to the lower surface of housing 14.Passages 78 and 80 are aligned respectively with inlet hole 66 andoutlet hole 68 in upper electrode 12 as shown in FIG. 2. The variouspassageway interfaces between housing 14 and upper electrode 12 aresealed by O-rings in a conventional manner. Cooling fluid thus flowsinto inner housing 14 through inlet passages 20, 74 and 78, then throughupper electrode 12 via inlet hole 66 and passage 56, and finally out ofhousing 14 through outlet hole 68 and passages 80, 76 and 22.

Baffle 16 is affixed to flange 82 of inner housing 14 intermediate upperelectrode 12 and the outlet of reactive gas passageway 18 as shown inFIG. 1. Baffle 16 preferably includes a plurality of holes therethroughsimilar in size and configuration to holes 54 in upper electrode 12.However, the smaller (0.1-0.4 mm.) openings are facing upwards in FIG.1, and the pattern of holes is arranged so that the holes in baffle 16are not directly aligned with holes 54 in electrode 12. Thisconfiguration creates a pressure differential between the upper andlower surfaces of baffle 16 and assures a uniform distribution ofreactive gas over the surface of upper electrode 12. Other bafflestructures may be used to create the desired pressure differential, forexample, a sintered baffle plate. However, a sintered baffle has thedisadvantage of lengthening the time required to stabilize pressureprior to etching and to purge the reactor of reactive gas after anetching operation is completed.

Upper electrode 12 and the upper section 40 of lower electrode 38 may befabricated from any conductive material that is compatible with thedesired reactive gas and other process parameters, for example,stainless steel, aluminum, copper or the like. The insulating elementsof plasma reactor 10, i.e., insulating housing 24, bushing 26, exhaustring 28, clamp ring 36, lower section 42 of lower electrode 38, andinsulating ring 44, may be plastic, nylon, glass ceramic, quartz,polytetrafluorethylene, or other suitable insulator. Baffle 16 can beone of the above conductive materials or sintered graphite. Innerhousing 14 is conductive and outer housing 30, which may be eitherconductive or non-conductive, may be formed of any of the aforementionedconductive or insulating materials, the selection depending upon thereactive gas and the desired structural integrity and ease offabrication. In the described embodiment, upper electrode 12, section 40of lower electrode 38, baffle 16, inner housing 14 and outer housing 30are aluminum. Insulating ring 44 is glass ceramic, section 42 of lowerelectrode 38 is plastic, and insulating housing 24, bushing 26, exhaustand spacing ring 28, and clamp ring 36 are nylon.

The present system achieves high plasma etch rates with uniform etchingover the entire surface of the wafer. Contributing to this are theuniform reactive gas distribution, the extremely close spacing of theelectrodes, and a high degree of plasma confinement. Since there are noconducting surfaces exposed to the upper electrode 12 other than thewafer, substantially all of the RF power from the plasma is dissipatedby the wafer, thus assuring a high etch rate.

Surface 41 of lower electrode 38 is slightly smaller than the wafer tobe etched, leaving a small overhang at the edge of the wafer. Further,because surface 45 of insulating ring 44 is slightly lower than surface41, the wafer rests entirely on surface 41 to ensure good electricalcontact between the wafer and the lower electrode. The verticaldisplacement between surface 41 and surface 45 is preferablyapproximately 1-2 orders of magnitude less than the gap between upperelectrode 12 and lower electrode 38.

The elimination of wafer clamping, which is used in various priorreactors, minimizes reactive gas flow and electron density distortionsat the wafer edge and makes available the entire surface of the waferfor integrated circuit fabrication.

It will be understood by those skilled in the art that the selection ofthe precise process parameters, i.e., reactive gas chemistry, RF powerlevel and frequency, gas pressure and flow rate, etch time, etc., willdepend upon the thickness and type of material to be etched. However,excellent results have been achieved with the present reactor using avariety of reactive gases for etching various materials. In oneembodiment, the RF power density is greater than about 3 watts/cm², anda high reactive gas pressure, in the range of about 0.5-10 torr, ismaintained in plasma region 46. The temperatures of upper electrode 12and lower electrode 38 are regulated by passing chilled watertherethrough, although other types of cooling fluid may also be used.The configuration of the present reactor, which permits the high RFvolume power density and gas pressure, results in a substantiallyincreased etch rate and etch uniformity as compared to prior reactors.This, in turn, decreases the required etch time, in many instances by anorder of magnitude or more.

The disclosed reactor system was used to etch such materials as dopedand undoped silicon, photoresists, polyimide and silicon dioxide. Etchrates of approximately 1.5 microns/min. for thermal SiO₂ in CF₄ and 4microns/min. for polymeric materials in oxygen have been achieved. Ofcourse, any material capable of being plasma etched can be processed bythe reactor.

The present plasma reactor thus provides a single wafer, high pressuresystem which exhibits improved etch rates and etch uniformity foretching a variety of materials.

Obviously, many modifications and variations of the disclosed reactorsystem will become apparent to those skilled in the art given thebenefit of the foregoing disclosure. It is to be understood that suchmodifications and variations may be made without departing from thespirit and scope of the present invention as defined by the appendedclaims.

What is claimed is:
 1. Apparatus for plasma etching a semiconductorwafer comprising:a fluid cooled lower electrode for supporting saidwafer; a fluid cooled upper electrode substantially parallel to andspaced apart from said lower electrode a distance such that the aspectratio is greater than about 25, said upper electrode including means fordistributing a reactive gas uniformly over said wafer; means forcoupling RF power to one or both of said electrodes; and means forconfining a plasma between said upper and lower electrodes.
 2. Theapparatus of claim 1 wherein said plasma confining means comprises:aninsulating ring surrounding a raised portion of said lower electrode,said insulating ring having an upper portion extending above the uppersurface of said lower electrode, and a channel formed into its innersurface such that the inner surface of said insulating ring ishorizontally and vertically spaced apart from the upper exposed surfaceof said lower electrode, the upper portion of said insulating ringfurther including a plurality of radially extending passagestherethrough for exhausting said reactive gas, wherein the inner surfaceof said insulating ring and the facing surfaces of said upper and lowerelectrodes form a plasma confinement region.
 3. The apparatus of claim2, further comprising:a first housing affixed to said upper electrode toform a plenum thereabove for said reactive gas, said first housingincluding a first passageway therethrough for communicating saidreactive gas to said plenum and a plurality of second passagewaystherein for communicating a cooling fluid to and from said upperelectrode; a second housing surrounding and spaced apart from said firsthousing, said second housing including reactive gas exhaust means; andan insulating housing intermediate said first and second housings toelectrically isolate said first housing from said second housing, theouter surface of said insulating housing being spaced apart from saidsecond housing to provide a passageway for said reactive gas from saidinsulating ring passageways to said second housing gas exhaust means. 4.The apparatus of claim 3, further comprising:baffle means affixed tosaid first housing in said plenum above said upper electrode touniformly distribute said reactive gas to said upper electrode.
 5. Theapparatus of claim 1 wherein said upper electrode reactive gasdistributing means comprises a plurality of vertical passagewaystherethrough, the width of each of said passageways decreasing from theupper surface to the lower surface of said upper electrode, and whereinsaid upper electrode includes an internal cooling passage to uniformlydistribute a cooling fluid therethrough.
 6. The apparatus of claim 1wherein said lower electrode includes an internal cooling passage touniformly distribute a cooling fluid therethrough, and an insulatedlower portion to electrically isolate said lower electrode.
 7. Apparatusfor plasma etching a semiconductor wafer comprising:a fluid cooled lowerelectrode for supporting said wafer; a fluid cooled upper electrodesubstantially parallel to and spaced apart from said lower electrode,said upper electrode including means for distributing a reactive gasuniformly over said wafer; means for coupling RF power to one or both ofsaid electrodes; and an insulating ring surrounding a raised portion ofsaid lower electrode, said insulating ring having an upper portionextending above the upper surface of said lower electrode, and a channelformed into its inner surface such that the inner surface of saidinsulating ring is horizontally and vertically spaced apart from theupper exposed surface of said lower electrode, the upper portion of saidinsulating ring further including a plurality of radially extendingpassages therethrough for exhausting said reactive gas, wherein theinner surface of said insulating ring and the facing surfaces of saidupper and lower electrodes form a plasma confinement region.
 8. Theapparatus of claim 7 wherein said upper electrode reactive gasdistributing means comprises a plurality of vertical passagewaystherethrough, the width of each of said passageways decreasing from theupper surface to the lower surface of said upper electrode, and whereinsaid upper electrode includes an internal cooling passage to uniformlydistribute a cooling fluid therethrough.
 9. The apparatus of claim 8wherein said lower electrode includes an internal cooling passage touniformly distribute a cooling fluid therethrough, and an insulatedlower portion to electrically isolate said lower electrode.
 10. Theapparatus of claim 9, further comprising:a first housing affixed to saidupper electrode to form a plenum thereabove for said reactive gas, saidfirst housing including a first passageway therethrough forcommunicating said reactive gas to said plenum and a plurality of secondpassageways therein for communicating a cooling fluid to and from saidupper electrode; a second housing surrounding and spaced apart from saidfirst housing, said second housing including reactive gas exhaust means;and an insulating housing intermediate said first and second housings toelectrically isolate said first housing from said second housing, theouter surface of said insulating housing being spaced apart from saidsecond housing to provide a passageway for said reactive gas from saidinsulating ring passageways to said second housing gas exhaust means.11. The apparatus of claim 10, further comprising:baffle means affixedto said first housing in said plenum above said upper electrode touniformly distribute said reactive gas to said upper electrode. 12.Apparatus for plasma etching a semiconductor wafer comprising:a lowerelectrode for supporting said wafer, said lower electrode having aninsulated lower portion and an internal cooling passage to uniformlydistribute a cooling fluid therethrough; an upper electrodesubstantially parallel to and spaced apart from said lower electrode,said upper electrode having an internal cooling passage to uniformlydistribute a cooling fluid therethrough and a plurality of verticalpassageways therethrough for distributing a reactive gas uniformly oversaid wafer, wherein the width of each of said vertical passagewaysdecreases from the upper surface to the lower surface of said upperelectrode; means for coupling RF power to one or both of saidelectrodes; and an insulating ring surrounding a raised portion of saidlower electrode, said insulating ring having an upper portion extendingabove the upper surface of said lower electrode, and a channel formedinto its inner surface such that the inner surface of said insulatingring is horizontally and vertically spaced apart from the upper exposedsurface of said lower electrode, the upper portion of said insulatingring further including a plurality of radially extending passagestherethrough for exhausting said reactive gas, wherein the inner surfaceof said insulating ring and the facing surfaces of said upper and lowerelectrodes form a plasma confinement region.
 13. The apparatus of claim12, further comprising:a first housing affixed to said upper electrodeto form a plenum above said upper electrode for said reactive gas, saidfirst housing including a first passageway therethrough forcommunicating said reactive gas to said plenum and a plurality of secondpassageways therein for communicating a cooling fluid to and from saidupper electrode; a second housing surrounding and spaced apart from saidfirst housing, said second housing including reactive gas exhaust means;and an insulating housing intermediate said first and second housings toelectrically isolate said first housing from said second housing, theouter surface of said insulating housing being spaced apart from saidsecond housing to provide a passageway for said reactive gas from saidinsulating ring passageways to said second housing gas exhaust means.14. The apparatus of claim 13, further comprising:baffle means affixedto said first housing in said plenum above said upper electrode touniformly distribute said reactive gas to said upper electrode.